Thank you everyone for your advice and support, and for keeping my Lily in your thoughts. Means a lot!

Lily had her surgery this morning. My vet picked her up at my home at 8:15 a.m. (my car broke last week and hasn't been fixed yet) after a rough night (Lily wasn't allowed to eat after 6 p.m. yesterday and was hungry, and Luna was confused about the fact she had been fed separately). I was staying at home with Luna.
Around 10:15 a.m., my vet called to tell me the surgery was done. The good news (in fact, awesome news as I didn't expect this!) - no tumor in her bladder! The bad news: there was nothing in her bladder that looked as if it could be responsible for her problems, so we still don't know what's bothering her. Last week, I have started a treatment with a pain med called Melosus, and she responded well to it, was more relaxed and - as far as I can tell - hasn't peed outside her litterbox, so it seems her spraying was caused by pain.
What's strange: We definitely saw something inside her bladder in the ultrasounds she had on Monday and Thursday last week, and it looked like a tumor. My vet said when she had opened her belly today, her bladder looked from outside as if there was something in it, but when she opened it, there was nothing. She took biopsies from the bladder tissue and now we have to wait for the result. Could be that there was some kind of inflammation that has healed in the meantime as a result from the Melosus treatment. The entire thing is a little strange - and makes me feel a little bad 'cause we've put her through that surgery without finding anything. On the other hand, it would have been worse if they'd found a tumor!

I got Lily back this afternoon, and she's miserable and upset. She will spend the night separated from Luna, I've made a "sick room" for her with many blankets and spots to hide. When I enter the room, she'd be hissing and growling at me, that's how miserable she is (and still under the influence of the anaesthesia, judging from the way her eyes look like), but I was able to calm her down, so she came over and gave me some - pretty wobbly - headbumpies. She also ate a little, an hour ago. If she keeps it down, I will offer her some more before I go to bed.

I hope she'll have a good night and will feel much better tomorrow. Oh, and btw, she already peed several times and used the litter box. I was surprised to see this was working so well, after all, her bladder has been opened, I don't even wanna imagine the pain and the discomfort! But it works, even though there's some blood in it (which is pretty normal due to the incision).

I will keep you updated!